I haven't read "Slaying the Dragon" yet (its on my to-read list), but . . . "forced"? I'm skeptical until I read Rigg's book. TSR telling Salvatore, "Hey, your dark elf character is the one everyone is responding to, we want more of that! And if you want to keep writing D&D novels for us . . . ."
All (official) D&D novels are work-for-hire, and to various degrees, each author gets a list of what TSR, then later WotC, wants out of the books. This was true with the Dragonlance novels all the way through to the movie tie-in novels released this week. Salvatore being "forced" to write about Drizzt isn't any more or less true than all of those other authors being "forced" to write towards TSR/WotC's specs.